How San Francisco 49ers' Chris Borland made his decision to retire
San Francisco 49ers Chris Borland retires over repeated head
49ers Chris Borland retires from football, citing concern over head
NFL: Players call it quits in prime of careers
NFL rookie Chris Borland on retiring over concussion concerns
Chris Borland, Fearing for Health, Retires From the 49ers. At 24
Chris Borland retires from NFL aged 24
49ers linebacker Chris Borland retiring at 24 due to long-term health
Is Chris Borland's retirement the beginning of the end for the NFL